An active-typed double balanced mixer is popular as one of frequency converters for a radio apparatus. Since the active-typed double balanced mixer consumes electricity in a mixer stage, it is difficult to achieve low power consumption. One of the frequency converters having lower power consumption is a passive-typed double balanced mixer. An example of the passive-typed double balanced mixer is disclosed in W. Redman-White, et al., “1/f noise in passive CMOS mixers for low and zero IF integrated receivers,” Proceedings of the 27th European Solid-State Circuits Conference, 2001, pp. 41-44.
However, in the passive-typed double balanced mixer disclosed in the above reference, input impedance assessed from the mixer stage decreases because of charging and discharging parasitic capacity of a switching transistor. In recent years, a radio apparatus which is capable of varying frequencies applied to a local signal has been known. In such the radio apparatus, the frequency of the local signal may vary, and such the local signal is inputted into the passive-typed double balanced mixer. As a result, the input impedance of the passive-typed double balanced mixer decreases depending on the frequency of the local signal.